i40e/i40evf: Fix casting in transmit code

Simple cast to fix a sparse warning.

Fixes: commit 5453205c ("i40e/i40evf: Enable support for
SKB_GSO_UDP_TUNNEL_CSUM")

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>